Allow to use old style menu alignment

Closes #97 (closed).

Definitely not obvious. We must at least keep the ability to switch to the native alignment, and for now I would say: keeping it as the default choice, and with only a hidden setting.

It is the current state of this MR: to activate old style menu, run

gsettings set org.xfce.mousepad.preferences.window old-style-menu true

I encountered several bugs or conflicts in the management of signals and references that I didn't fully elucidate, even if I finally found a combination of APIs that works in all cases in the current state of menus and actions in Mousepad (problems occur especially with radio actions, I have tried to comment all this correctly). So it is not at all excluded that bugs may occur in the future.

Otherwise it doesn't seem to have too much of an impact on performance, even if there is some extra work that is done each time a menu is updated.

It did in fact have an impact on performance, but a second method was proposed that greatly reduces this impact and avoids all the problems mentioned above: see !77 (comment 25067).

As this second method introduces a small delay in the display of some menus, a third method has been developed that combines the first two: see !77 (comment 25197).